Vacuum method



June 28, 1966 AMES ETAL 3,258,193

VACUUM METHOD Filed April 27, 1959 2 Sheets-Sheet 1 INVENTORS 1 IRVING AMES ROBERT L. CHRISTENSEN ATTORNEY 1. AMES ETAL June 28, 1966 VACUUM METHOD 2 Sheets-Sheet 2 Filed April 2'7, 1959 FIG.3

United States Patent Ofilice 3,258,193 Patented June 28, 1966 3,258,193 VACUUM METHOD Irving Ames, Peekskill, Robert L. Christensen, Poughkeepsie, and Jack Teale, Pleasant Valley, N.Y., assignors to International Business Machines Corporation,

New York, N.Y., a corporation of New York Filed Apr. 27, 1959, Ser. N 0. 809,049 13 Claims. (Cl. 230-69) This invention relates to a method of attaining a vacuum and, more particularly, to a method of attaining a vacuum without the use of pump fluids.

Generally, most vacuum chambers are evacuated by means of one or more pumps wherein a fluid is employed in the system being evacuated. This fluid may be a working fluid as in diffusion pumps, a sealing fluid :as in conventional rotary mechanical pumps, or as a lubricant in oil-free pumps. It has been found that significant quantities of vapors of these fluids enter the working chamber and impart undesirable characteristics to the working surfaces therein. available various types of pumps which do not introduce contaminating vapors into the vacuum system. An eX- ample of a non-contaminating pump is the now wellknown electronic high vacuum pump which does not employ a pump fluid. However, a serious disadvantage of non-contaminating pumps results from the fact that, in general, they are not operable until the pressure within the vacuum system has initially been reduced below atmospheric. Thus, it has been necessary until now, to employ at least one of the above described pumps employing a Recently, there have become pump fluid prior to employing a non-contaminating pump.

What has been discovered is a rapid and efiicient method of reducing the pressure within a vacuum chamber below atmospheric without the us of pump fluids. This novel approach to vacuum technique employs a noncontaminating liquid helium pump. By means of the helium pump, the pressure Within a chamber may be reduced from atmospheric to 10 mm. Hg or less. The method consists, essentially, of connecting a condensation chamber to a vacuum chamber and thereafter cooling the condensation chamber with liquid helium. Although helium cooled surfaces have hitherto been employed as cold traps and as :a means for attaining eX- tremely low pressures in vacuum chambers once they have been evacuated by the hereinbefore mentioned fluid pumps, the novel method of the invention employs a helium chamber as a pump capable, of and by itself, of reducing the pressure within a vacuum chamber below atmospheric. By means of one or more non-contaminating pumps, high or ultra-high vacua may be attained wherein no contaminating vapors are introduced into the .vacuum chamber by the pumping means.

Additional features of the invention include the lack of moving parts and the vibration and maintenance problems associated therewith as well as providing a pump lighter in Weight than the usual vacuum pump.

A primary advantage of the liquid helium pumping method is the inherent cleanliness and lack of contamination introduced by the-method. The cleanliness of surfaces within'the vacuum chamber is limited only by contamination released from the component structures of the vacuum system itself.

An object of the invention is to provide an improved method of attaining a vacuum.

A further object of the invention is to provide a method of attaining a vacuum without the use of pump fluids.

Still another object of the invention is to provide a method of attaining a vacuum without the use of moving parts.

Yet another object of the invention is to provide a non-contaminating pump operable at atmospheric pressure.

Another object of the invention is to provide a method of attaining a pressure whose lower limit is set only by the sizes and vapor pressures of the components employed in the system.

A still further object of the invention is to provide a method of attaining a vacuum employing a helium pump.

Yet another object of the invention is to provide a method of attaining a vacuum employing a helium pump as a roughing pump.

The foregoing and other objects, features and advan-. tages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings.

In the drawings: FIG. 1 is a cross-sectional view of a helium pump. FIG. 2 is a diagrammatic section of an apparatus that may be employed in practicing the method of the invention.

FIG. 3 illustrates a modification of the apparatus of FIG. 2.

FIG. 1 illustrates a helium pump which may be employed in practicing the method of the invention. As shown in FIG. 1, a pump, 1, includes three chambers. In operation, a chamber 2 is filled with liquid nitrogen and a chamber 3 is filled with liquid helium. Gases from the system being evacuated are then collected in a condensation chamber 4 which is connected to a vacuum chamber (not shown) by means of an opening 5. Thus, the pressure within the vacuum chamber is reduced from atmospheric by condensing out all gases and vapors of higher boiling point on the surface of chamber 4 which is maintained at 42 K. by the liquid helium in chamber 3. The residual pressure in the system is then due to various gases en route to the surface of chamber 4 and to the few parts per million of helium present in the atmosphere at sea level. This residual pressure can be further reduced by reducing the pressure operable on the surface of the liquid helium, by means of conventional vacuum pumps, to thereby lower its boiling point and condense some of that gas from within the system.

The method of the invention may be practiced with various types of apparatus. FIG. 2 discloses, by way of example, an apparatus which has satisfactorily been employed to obtain the advantages of the method of the invention. As shown in FIG. 2, a vacuum chamber 10 is connected to a liquid helium pump 1 by means of tubing 12. The pump 1 contains a receptacle for liquid helium and a condensation chamber, both of which are surrounded by a second receptacle for liquid nitrogen, as hereinbefore described. Connected in series with the tubing 12 are a pair of valves 16 and 17 constructed of low vapor pressure materials. A Pirani vacuum gauge 18 is attached to the tubing between these valves. Additionally, a Bayard-Alpert ion gauge 19 is attached to the main vacuum chamber 10 through a side arm 20. The Pirani vacuum gauge 18 is used to measure pressures down to about 10- mm. Hg and the ion gauge 19 is employed to measure pressures down to about 10 mm. Hg. The use of the valves 16 and 17 will be hereinafter described in detail. Valve 21, similar to valves 16 and 17, is employed to connect the system to atmospheric pressure.

In general, the apparatus of FIG. 2 is employed in practicing the method of the invention in the following manner. The specimen or device which is to be examined or operated in a vacuum is placed within the vacuum chamber 10. Valves 16 and 17 are next opened and valve 21 is closed. Liquid nitrogen is then caused to fill receptacle 2 after which liquid helium is admitted to receptacle 3 (see FIG. 1). The liquid nitrogen is used merely to reduce the evaporation rate of the liquid helium. The pressure within vacuum chamber is reduced as soon as receptacle 4 (see FIG. 1) is sufficiently cooled.

.In several minutes, the residual pressure within vacuum chamber 10 is measured to be in the order of about 10* mm. Hg.

Although a pressure of about 10 mm. Hg is adequate in many processes, the method of the invention can also be utilized to obtain high vacua; that is, pressures in the range of 10"= to 10 mm. Hg while at the same time maintaining the advantages afforded by the method of the invention. As described above, the liquid helium pump is first utilized to reduce the pressure within vacuum chamber 10 to about 1 micron. Following this, the next step in the method is to operate a second non-contaminating pump, which may be, by way of example, an electronic pump or another helium pump, close valve 16 and then continue pumping until the desired pressure is attained.

If ultra-high vacua, that is, pressures below 10- mm. Hg are desired, the vacuum system is Subjected to a bake-out in order to free a large amount of absorbed vapors from various components of the system. This bake-out is first performed with valves 16 and 17 open, in order that these liberated vapors will be collected by the liquid helium pump. If this procedure is employed, valve 16 should, in addition to being made of low vapor pressure material, be bakeable. This bake-out is maintained for a time sufficient to free a majority of the aJbsorba-tes adhering to the walls of the vacuum systern. At this point, valve 17 is closed, and a non-contaminating pump which may be an electronic pump 22 as shown in FIG. 2, connected to vacuum chamber 10 by means of the vacuum seal 23, is then-operated. Electronic pump 22 may be selected from the class of recently developed vacuum pumps which operate through the combined effect of ionization of the residual gas molecules within a vacuum chamber, and gettering of the residual gas molecules by means of a freshly created getter surface. Pump 22, therefore, does not impair the cleanliness within vacuum chamber 10 by introducing vapors of a pump fluid. Because present electronic pumps cannot be placed in operation until the working pressure is reduced below about 20 microns, the use of a liquid helium pump, according to the method of the invention, is particularly complementary to an ion pump for obtaining Vacua without the use of pump fluids. A satisfactory model of an ion pump is presently marketed by Varian Associates, Inc., Palo Alto, California, under the trademark Vacion as Model VA-1402 High Vacuum Pump.

Upon firing of the ion pump, the bake-out may be continued, if desired, as follows. The oven temperature is increased as the pressure within chamber 10 is reduced. The increased temperature is maintained for a time sufiicient for the ion pump to reduce the pressure within vacuum chamber 10 to between about 10 to 10- mm. Hg. Upon cooling, bake-out valve 16 is closed, and by means of continued ion pumping, the resultant pressure may be reduced to the order of 10" mm. Hg.

As an aid in understanding the invention, the following specific values are presented, by way of example, but it should be understood that variations in these specific values may be employed by those skilled in the art while maintaining the advantages afforded by the heretofore described method. Vacuum chamber 10 consists of a 1 /2 inch diameter glass manifold about 18 inches long, having a volume of about 1 liter. Receptacle 3 (see FIG. 1) also has a volume of about 1 liter, with about 50 square inches of the surface area of the condensation chamber 4 exposed to the liquid helium. After the liquid helium pump has reduced the pressure within vacuum chamber 10 to about 1 micron, an oven 24 subjects the system to a temperature of about 250 C. for 3 hours in order to drive off a large amount of loosely sorbed vapors from the walls of the manifold. The next. step in the method then consists of closing bake-out valve 17, firing ion pump 22, and heating the system to about 450 C. Valve 16 remains open while it is hot, in order to prevent its being diffusion-welded closed. This is the reason for connecting valve 17 in series with valve 16. The 450 C. temperature is maintained for about 6 hours by which time the ion pump has reduced the pressure to between 10" to 10" mm. Hg. After the oven and system have cooled suificiently, bake-out valve 16 is closed, thus isolating the chamber 10 from the liquid helium pump. 1. Finally, ion getter pumping for several more hours reduces the pressure within chamber 113 to about 10- mm. Hg.

It has been found that this method results in a reduction by a factor of at least 50 in the hydrocarbon vapors remaining in such a vacuum as compared to the same vapors in a vacuum of the same pressure produced by a conven tional vacuum system employing an oil diffusion pump and a liquid nitrogen cold trap.

The volume of a vacuum chamber which can be evacuated by means of a liquid helium pump from atmospheric pressure to a pressure in the order of 10- mm. Hg is, of course, limited by the area of condensation chamber 4 which is cooled by the liquid helium. However, it has been found that the principles of the invention may be applied to large vacuum systems without employing extremely large helium condensation pumps, by initially employing a water aspirator to remove up to 99% of the gases within the vacuum system, and thereby to reduce the pressure to a few mm. Hg, prior to helium pumping.

FIG. 3 illustrates the system of FIG. 2 with the addition of a water aspirator 25 connected to tubing 12 by means of bake-out valve 26. As shown in FIG. 3, the water aspirator 25 lowers the pressure in chamber 10 and con densation chamber 4 through tubing 12 and open valves 16, 17 and 26. Valve 26 is then closed and the hereinbefore described steps of the method of the invention may be employed to further lower the pressure. It will be understood by one skilled in the art, that the pressure attained by the water aspirator determines the required surface area of the condensation chamber and therefore the lower the pressure attained by the water aspirator, the smaller the surface area of the condensation chamber necessary to attain the pressure at which a non-mechanical pump can be effectively employed. Or, conversely, for a given area of condensation, a lower pressure attained by the Water aspirator permits a correspnodingly larger volume of chamber 10 to be evacuated by the liquid helium.

Because of the possibility of pressure build-up in condensation chamber 4 when the liquid helium pump is iso lated from the vacuum chamber 10 by means of valves 16 and 17, valve 21 may be used to reduce the pressure therein to atmospheric. After the liquid helium in receptacle has evaporated, the pressure built up in atmospheres in chamber 4, if a water aspirator has not been used intially in the pumping sequence, is approximately the ratio of the volum of the vacuum system to the volume of the condensation chamber. This pressure can be released by means of valve 21. In this manner, cycling a small liquid helium pump is possible so that the pump can be used, in steps, to evacuate a large volume. Alternately, a helium refrigerator may be employed as a means of cooling condensation chamber 4 continuously in order to avoid pressure build-up therein.

As will be understood by one skilled in the art, the method of the invention is adaptable for many applications such as vacuum deposition of thin films, vacuum melting processes, evacuation of particle accelerators, vacuum tubes, mass spectrometers, electron microscopes and the like.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. The method of preparing a vacuum chamber for pumping by a pump which is capable of further reducing th pressure within said chamber after the pressure therein has initially been reduced to a pressure in the order of mm. Hg comprising the step of operating a cryogenic liquid condensation pump for a time sufficient to reduce the pressure within said chamber from atomspheric to less than l0 mm. Hg, and thereafter operating said pump to further reduce pressure within said chamber.

2. The method of preparnig a vacuum chamber for pumping by an electronic pump consisting of operating a cryogenic liquid pump as a roughing pump for a time sufiicient to lower the pressure within said chamber to a value at which said electronic pump is capable of commencing pump operations, and thereafter operating said electronic pump to further reduce pressure within said chamber.

3. The method of reducing the pressure from atmospheric to below 10- mm. Hg within a vacuum chamber having a condensation chamber attached thereto consisting of cooling said condensation chamber with cryogenic liquid to lower the pressure within said vacuum chamber by condensing gas molecules of said vacuum chamber upon the walls of said condensation chamber, and thereafter operating a vacuum pump to further reduce pressure within said vacuum chamber.

4. The method of attaining an ultra-high vacuum within a vacuum chamber having a condensation chamber and a non-contaminating pump attached thereto comprising cooling said condensation chamber with cryogenic liquid for a time sufiicient to condense a portion of the gas molecules of said vacuum chamber within said condensation chamber whereby a pressure of less than 10 mm. Hg is attained within said vacuum chamber, and thence operating said non-contamination pump to attain an ultrahigh vacuum.

5. Means for producing high vacuum comprising a cryogenic pump coupled to a vacuum chamber which is to be evacuated to a high vacuum, said cryogenic pump having an inner container holding a first cryogenic liquid and a double-walled outer container, the space between the double walls of the outer container being filled with a second cryogenic liquid, said double-walled outer container being positioned about said inner container to materially reduce the amount of radiant energy striking said inner container directly from outside said double-walled outer container, said double-walled container including an opening providing communication between the vacuum chamber to be evacuated and the space defined by the inner wall of the double-walled container so that gas molecules can diffuse from said vacuum chamber into said space for sorption on the surface of the inner container.

6. Means for producing high vacuum comprising a cryogenic pump coupled to a vacuum chamber, said cryogenic pump having an inner container holding a lowboiling cryogenic liquid and an outer container in heat exchange relationship with a higher-boiling cryogenic liquid, said outer container being positioned about said inner container to materially reduce the amount of radiant energy striking said inner container directly from outside said outer container, said container including an opening providing communication between the vacuum chamber and the space defined by the inside of the outer container so that gas molecules can difluse from said vacuum chamber into said space for sorption on the colder inner container.

7. The method of claim 3 wherein the volume of said vacuum chamber is approximately equal to the volume of said condensation chamber.

8. The method of attaining an ultra-high vacuum without the use of pump fluids within a vacuum chamber having an electronic pump and a helium pump attached thereto comprising; operating said helium pump for a time suificient to reduce the pressure within said chamber below 10- mm. Hg; operating said electronic pump; subjecting said chamber and said electronic pump to a predetermined temperature to remove a portion of sorbed gases therefrom; isolating said helium pump from said chamber; cooling said chamber and said electronic pump; and continuing said electronic pumping until said ultrahigh vacuum results.

9. The method of preparing a vacuum chamber for pumping by a pumpwhich is capable of reducing the pres-sure in said chamber to a pressure of the order of 10 mm. Hg after the pressure therein has initially been reduced to a pressure of the order of 10* mm. Hg comprising the steps of; operating a water aspirator for a first time interval suflicient to reduce the pressure within said chamber to a few mm. Hg; isolating said Water aspirator from said chamber; and thereafter operating a helium pump for a second time interval suflicient to further reduce the pressure to less than 10 mm. Hg.

10. The method of reducing the pressure from atmospheric to below 10* mm. Hg within a vacuum chamber having a condensation chamber attached thereto comprising; first reducing the pressure within said vacuum chamber by means of a water aspirator; and thereafter cooling said condensation chamber with liquid helium to lower the pressure within said vacuum chamber by condensing gas molecules of said vacuum chamber upon the walls of said condensation chamber.

11. The method of claim 10 wherein the volume of said vacuum chamber is substantially greater than the volume of said condensation chamber.

12. The method of attaining an ultra-high vacuum within a vacuum chamber having a condensation chamber and a non-contaminating pump attached thereto comprising the steps of; reducing the pressure within said chamber by means of a water aspirator; isolating said water aspirator from said chamber cooling said condensation chamber with liquid helium for a time sufiicient to condense a portion of the molecules of said vacuum chamber within said condensation chamber whereby a pressure less than 10- is attained; and thence operating said non-contaminating pump to attain an ultra-high vacuum.

13. The method of reducing the pressure from atmospheric to below 10* mm. Hg within a vacuum chamber having a condensation chamber attached thereto comprising the steps of; removing up to 99% of the gases within said vacuum chamber; and thereafter cooling said condensation chamber with liquid helium for several minutes, whereby a pressure less than l0 mm. Hg is attained.

References Cited by the Examiner UNITED STATES PATENTS 2,796,555 6/ 1957 Connor. 2,855,140 10/1958 Sedlacsik 230-101 2,985,356 5/1961 Beecher 230-69 3,009,629 11/1961 Garin et a1 230-69 FOREIGN PATENTS 1,180,717 l/1959 France.

MARK NEWMAN, Primary Examiner.

JOSEPH H. BRANSON, JR., LAURENCE V. EFNER,

Examiners. W. E. COLEMAN, Assistant Examiner. 

1. THE METHOD OF PREPARING A VACUUM CHAMBER FOR PUMPING BY A PUMP WHICH IS CAPABLE OF FURTHER REDUCING THE PRESSURE WITHIN SAID CHAMBER AFTER THE PRESSURE THEREIN HAS INITIALLY BEEN REDUCED TO A PRESSURE IN THE ORDER OF 10-2MM. HG COMPRISING THE STEP OF OPERATING A CRYOGENIC LIQUID CONDENSATION PUMP FOR A TIME SUFFICIENT TO REDUCE THE PRESSURE WITHIN SAID CHAMBER FROM ATMOSPHERIC TO LESS THAN 10-2MM. HG, AND THEREAFTER OPERATING SAID PUMP TO FURTHER REDUCE PRESSURE WITHIN SAID CHAMBER. 